Mechanical Properties of Unoriented Films

Samples used for the study of the structural transformations occurring during stretching and for the mechanical tests have been prepared by compression molding. The X-ray powder diffraction profiles of melt-crystallized 

The iPP sample of lowest stereoregularity (sample iamPP) does not crystallize by cooling the melt to room temperature, but slowly crystallizes in disordered modifications intermediate between a and 7 forms (Fig. 17.3B), if the sample, cooled from the melt, is kept at room temperature for several days [32]. In fact, the X-ray diffraction profile of sample iamPP of Fig. 17.2g 

The relative amount of 7 form, with respect to the a-form in the melt crystallized samples of Fig. 17.2 is reported in Fig. 17.4 as a function of the concentration of rr defects. The most isotactic sample crystallizes basically in the a-form (Fig. 17.2a), with a Umit low concentration of 7 form of 15 20%. The amount of 7 form increases with increasing content of rr defects up to 100% for rr concentrations higher than 6 7% [17,18,32]. 

The degree of crystallinity (Fig. 17.4) decreases only slightly with increasing concentration of rr defects in the range 0 11%, then drops to very low values for the less stereoregular sample iainPP. 

The stress-strain curves of compression-molded films of iPP samples of Table 17.1 are shown in Fig. 17.5. The values of the most important mechanical parameters are reported in Fig. 17.6 as a function of the concentration of rr defects. The values of Young modulus decrease with increasing concentration 

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